This invention relates to electrography and in particular relates to an electrostatic duplicator adapted to produce copies or prints of a subject matter in multiplicity of colors wherein all such colors are deposited on or transferred onto the copy in exact register relative to each other.
It is known to produce color prints by electrophotographic processes, such prints being commonly used as lithographic or gravure pre-press proofs. Such pre-press proofing processes are disclosed for instance in U.S. Pat. Nos. 3,337,340, 3,419,411 and 3,862,848. One such process, known as the Remak process, is described in addition in "The Printing Industry", by Victor Strauss, page 307, published 1967 by printing Industries of America Inc.
It is customary to produce such pre-press proofs by charging a photoconductive recording member followed by in register exposure through a separation transparency corresponding to one color, followed by toning of the exposed photoconductor with a liquid dispersed toner of the appropriate color, followed by in register transfer of the color toner image deposit to a receiving member surface, such as a printing stock.These process steps are repeated with appropriate color separation transparencies and color toners to produce prints with two, three or four colors, as required. The printing stock is usually dried after each transfer operation to prevent back transfer of previously applied color deposits, but is normally rewetted with clean toner dispersant prior to each transfer operation.
In the known electrophotographic color proofing processes operating in automatic equipment the time required per color production on the printing stock is in the range 4 to 7 minutes, that is 16 to 28 minutes are needed to produce one 4-color proof. This is because for each color production the aforesaid steps of charging, exposure, toning, transfer, paper pre-wetting and drying have to be repeated.
In many instances as is well known in the art it is required to produce several pre-press color proofs of one and the same subject, most commonly up to 10 proofs in lithography and over 20 proofs in gravure printing. It is also customary to produce so called progressives, that is pre-press proofs of all single colors, two colors and three colors of one and the same subject. In such instances the presently known electrophotographic color proofing processes are found to be excessively time consuming, because to produce for instance 10 4-color proofs the time required is 2 hours and 40 minutes to 4 hours and 40 minutes. Thus there is need for a process whereby a multiplicity of color proofs of one and the same subject can be produced for economic reasons in considerably shorter time than presently possible.
As the time of 16 to 28 minutes required to produce one 4-color proof in presently known automatic equipment can not be readily shortened in view of the aforesaid process steps involved, it would be obvious to one skilled in the art to employ the method of electrostatic duplication for speeding up the process in all those instances where a multiplicity of 4-color proofs and/or progressives of one and the same subject is required.
Electrostatic duplication is well known wherein it is customary to employ an electrostatic master having a printing surface which comprises relatively conductive background or non-image areas and relatively insulative or dielectric image or printing areas corresponding to the information to be printed. The conductive or relatively conductive background area may be the surface of a metal plate or coated paper or a photoconductive recording member or the like whereas the image areas which will be referred to henceforth as primary image deposits can be formed by deposits of dielectric material. In the process of duplication such electrostatic master is usually placed over a conductive support member such as a rotating cylinder which is connected to the terminal of one polarity of a high tension DC power supply whereas the terminal of the other polarity of such power supply is connected to corona or electrostatic field generating means positioned near the printing surface of said electrostatic master to apply electrostatic charge to the primary image deposits contained thereon. Subsequently color toner material henceforth referred to as secondary toner, is applied to the printing surface. Such toner material is attracted to the charged primary image deposits and forms color toner deposits thereon which will be referred to henceforth as secondary image deposits. Such secondary image deposits are then transferred electrostatically or by other means onto a receiving member such as a sheet of paper on which such transferred secondary image deposits are subsequently affixed by heat fusion or other means. Subsequent copies are generated in like manner by preferably cleaning the electrostatic master, recharging the primary image deposits, forming secondary image deposits thereon and transferring the secondary image deposits to consecutive copy sheets.
It would be obvious to apply electrostatic duplication to the production of a multiplicity of proofs of one and the same subject by firstly preparing electrostatic masters representing each color separation and then employing such masters in an electrostatic duplicator sequentially with secondary toners of appropriate colors to produce at relatively fast rate a desired number of multicolor proofs.
The electrostatic masters can be prepared conveniently in an automatic electrophotographic equipment of the kind previously referred to by charging the photoconductor, exposure in register to the first color separation, toning the photoconductor with a dielectric primary toner, transferring in register the dielectric toner deposits onto a master base such as metal sheet or conductive coated paper and affixing the primary dielectric toner deposits thereto by air or heat or other means to form the electrostatic master for the first color. Electrostatic masters for the second and subsequent colors would be produced in the same manner by employing for in register exposure the second and subsequent color separations. The same dielectric toner would be used for all masters.
Color proofs could then be produced in an electrostatic duplicator of the type described in the foregoing by mounting the first color electrostatic master on the cylinder and preparing the required number of proofs on printing stock with the first color secondary toner, then replacing the electrostatic master with that for the second color whilst also replacing the printing stock already containing the first color image thereon and printing with the second color secondary toner. Subsequent colors would be printed in like manner by sequentially replacing the electrostatic masters for the following colors, replacing the printing stock and employing secondary toners of appropriate colors.
In an alternative method the electrostatic masters for all colors could be simultaneously mounted circumferentially on one and the same cylinder having fitted thereto suitably positioned or activated applicators for secondary toners of appropriate colors corresponding with the respective color electrostatic masters to be toned. The color proofs would then be produced by printing sequentially all colors onto the printing stock the required number of times.
Both above methods of duplication have certain disadvantages.
Firstly, the printing stock must be placed in very precise register preparatory to the transfer thereto of a secondary color toner deposit, then moved out of register for drying and replaced again in register for re-wetting and transfer thereto of subsequent secondary color toner deposits. It is very difficult to maintain good registration with printing stock paper particularly of the inexpensive publication type in view of the poor dimensional stability of such papers, which, in addition are to be wetted with a solvent, dried and again re-wetted several times.
Secondly, all functions such as charging the electrostatic master, toning with secondary color toners, transfer onto printing stock and cleaning of the master must be carried out at one and the same speed, which is the circumferential speed of the rotating cylinder. In practice it is found in many instances that for best proof quality it is necessary to carry out the said functions at different speeds.
Thirdly, all functions are performed sequentially with consecutive color electrostatic masters and thus no time can be saved by overlapping some or all of the functions, that is to say it is not possible to simultaneously process electrostatic masters for two or more colors, nor to start making the next proof before the preceding one is completed.